A conventional boiling water reactor (BWR) is contained in a reactor building which includes a containment vessel for preventing significant fission product release in the event of an accident. The containment vessel is typically a reinforced concrete containment vessel (RCCV) having a steel liner as a leakage barrier for containing radiation therein. The vessel walls are typically about two meters thick, and with the steel liner, are conventionally sized for containing a maximum pressure therein of about 55 psig (3.9 kg/cm.sup.2), for example. The actual pressure within the containment vessel relative to the maximum design pressure of the containment vessel, e.g. 55 psig (3.9 kg/cm.sup.2), is conventionally known as the pressure margin. As the pressure within the containment vessel increases during operation, the pressure margin therein necessarily decreases. Suitable margin must be maintained during all modes of operation including expected accidents which requires that the containment vessel have a predetermined thickness and pressure containing capability.
Typically disposed inside the containment vessel and circumferentially around the reactor is a conventional wetwell or pressure suppression pool containing demineralized water. Defined between the reactor and the wetwell within the containment vessel is a conventionally known drywell which is simply an air space or chamber which, during normal operation of the reactor, contains a gas such as air or nitrogen. During an accident event such as a loss-of-cooling-accident (LOCA), steam is discharged from the reactor pressure vessel or main steam line into the drywell and is conventionally channeled through a plurality of conventional horizontal vents which are disposed in flow communication with the wetwell pool. The released steam is condensed in the wetwell pool which, therefore, reduces its pressure for maintaining acceptable pressure margin.
A conventional BWR includes a substantial number of active systems which require external power for controlling operation of the reactor and the many safety systems therefor. In order to reduce the number of active systems, a simplified boiling water reactor (SBWR) is being developed which will have a power output of about 600 MWe, for example. The SBWR is being designed to use natural circulation and passive features to minimize dependence on mechanical components and operator action, especially for operation of standard safety features.
For example, the SBWR also includes a containment vessel having a wetwell or pressure containment pool to absorb energy releases from the pressure vessel. However, the SBWR wetwell is an enclosed annular chamber having a top or roof which is commonly known as a diaphragm floor in the form of a relatively thick steel enclosed concrete slab which provides the top enclosure for the wetwell as well as provides a floor for the elevated pool of the gravity driven cooling system (GDCS) disposed thereon and a floor to support drywell piping and equipment. In an emergency, the SBWR vessel is depressurized, and cooling water flows by gravity from the GDCS pool into the reactor vessel to cool the reactor core.
The volume of the SBWR reactor building is generally not less than the volume of conventional BWR reactor buildings for a comparable power output. However, a smaller reactor building is desirable to reduce its volume and therefore reduce material and construction costs, and also reduce the time of construction to provide substantial cost savings in the building of the plant.
Accordingly, the SBWR reactor building preferably has a predeterminedly, relatively small volume wherein the containment vessel has a predetermined outer diameter and height preselected for containing all required equipment and for obtaining acceptable performance of the various systems associated therewith. For example, the drywell defined within the containment vessel has a predetermined volume which is specifically related to the volume of the wetwell air chamber above the pool of water therein. During a LOCA, for example, steam released into the drywell is channeled through the horizontal vents into the wetwell pool for being condensed therein. The steam, however, carries with it a portion of the air within the drywell which is compressed within the wetwell air chamber above the wetwell pool as it accumulates therein. The boundary defining the wetwell, including the diaphragm floor and drywell, must, therefore, be suitably configured and sized for accommodating the expected pressures therein, and is typically designed for containing pressures up to about 55 psig (3.9 kg/cm.sup.2), for obtaining a suitable pressure margin over the designed-for pressures therein. For the 600 MWe SBWR, the diaphragm floor has a thickness of about 1.6 meters and is encased within a steel liner, with the wetwell outer wall and floor being part of the containment vessel which is about 2 meters thick and steel lined. The wetwell inner wall is similarly sized for accommodating the expected pressures within the wetwell.
The wetwell is partially filled with water to a predefined level above the tops of the horizontal vents to provide acceptable operation thereof as is conventionally known. Accordingly, the wetwell air chamber above the wetwell pool has a predetermined volume which is preselected for accommodating the air entrained steam which is channeled through the horizontal vents into the wetwell pool, with the entrained air being compressed in the wetwell chamber during the LOCA, for example. This predetermined volume is constrained by the surfaces defining the wetwell air chamber which limits the pressure margin that may be obtained.
One manner of improving pressure margin in the wetwell during the LOCA, for example, would be to simply increase the size of the wetwell air chamber by increasing the height of the wetwell walls. However, this would also increase the height of the reactor vessel as well as that of the containment vessel, including the drywell, which would reduce the expected gain in pressure margin, as well as increase building materials and cost, and possibly penalize other plant systems which are related to building size. Similarly, if the wetwell air chamber diameter were increased, the containment vessel diameter would also increase which would increase the volume of the drywell. This, in turn, would lead to a further increase in wetwell pressure during the LOCA, and, therefore, less pressure margin.